Research by officials from the University of Cambridge Examinations Syndicate shows that autumn-born pupils score marks 2 to 3 per cent higher than those born in the summer.
A series of studies has found that summer-born pupils lag behind perhaps because they often start school at Easter, two terms after everyone else or in September when they are just four and struggle to keep up.
The researchers, who looked at results of 20,000 16-year-old pupils in science, 68,000 in maths and 1,679 in English, show that the effect lasts well beyond primary school.
They say that summer-born pupils are less likely to be entered for the more difficult levels in GCSE subjects where papers are divided according to ability.
Among the weakest pupils, the summer-born are more likely not to be entered for GCSE exams at all.
Alf Massey, head of the syndicate's research and evaluation division and one of the researchers, said: "It seems to me very interesting that you can detect such strong effects of birth-date in the data for 16-year-olds.
"The differences caused by birth-date are as large as those caused by sex but we are concerned about the latter but not the former."
He urged secondary schools to make allowances for children's birth-date when children were being placed into sets and when teachers were deciding which level of paper they should sit at GCSE
The research paper, by Mr Massey, Gill Elliott and Emma Ross in the journal Research Papers in Education, suggests that teachers may be misled by the apparent immaturity of some pupils at 16, causing them to enter the pupils for papers which were too easy. It says that teachers' expectations of summer-born children may be too low.
It also points out that they fall furthest behind their peers in teacher- assessed course work and essay questions and are at less of a disadvantage in shorter questions.
However, the researchers argue that changing exams to offer pupils a bigger choice of types of tasks would not be easy.
Government exam advisers have agreed that results of an optional national reading test for seven-year-olds should be adjusted to allow for age and given alongside the unadjusted result but the paper says that the introduction of similar measures at GCSE on which entry to employment and further education depends would be too contentious.
Mr Massey said parents of summer-born children should be aware of research findings and should consider carefully when it was appropriate for their children to start school. "Parents naturally want to get gifted children into schools early but there may be a case for delaying the start of school for those who are later developers. Some children born towards the end of the school year may be advised to wait until the beginning of the next school year."
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